There is conventionally known a sidelight type backlight device (see, for example, Patent Document 1).
Patent Document 1 mentioned above discloses a backlight structural body (backlight device) of a sidelight type (referred to also as an edge-light type) including a light guide plate as well as an LED light source and a reflective structure that are disposed on a lateral side of the light guide plate.
In this backlight structural body, the light guide plate is formed so that its light incidence surface and its light output surface are orthogonal to each other. Light outputted from the LED light source becomes incident on the light incidence surface of the light guide plate by being reflected by the reflective structure and then is outputted from the light output surface.
There is also conventionally known a sidelight type backlight device including a light guide plate having an inclined surface inclined with respect to a light output surface.
FIG. 10 is a cross-sectional view showing the structure of a display device provided with a sidelight type backlight device according to a conventional example. As shown in FIG. 10, a display device 501 provided with a sidelight type backlight device 510 according to a conventional example includes a display panel 502, an upper polarization plate 503 and a lower polarization plate 504 sandwiching the display panel 502 therebetween, the backlight device 510 disposed on a back surface side (lower side) of the display panel 502, and a metal frame 505 covering a back surface side portion of the backlight device 510.
The display panel 502 includes two transparent substrates 502a and 502b. Liquid crystal, which is not shown, is sealed in between the two substrates 502a and 502b. Furthermore, a driving IC (integrated circuit) 506 and an FPC (flexible printed circuit) 507 are attached to the front surface of the substrate 502a. The FPC 507 is bent at a predetermined position thereof so as to be disposed also on a back surface side of an after-mentioned frame 515.
The backlight device 510 includes a plurality of LEDs 511, a light guide plate 512 having a light incidence surface 512a on which light from the LEDs 511 becomes incident, a plurality of optical sheets 513 disposed in opposition to a light output surface 512b of the light guide plate 512, a reflection sheet 514 disposed on a back surface side of the light guide plate 512, and the frame-shaped frame 515 that houses these components.
The plurality of LEDs 511 are fixed to the FPC 507 via a solder layer 516 and disposed so as to be opposed to the light incidence surface 512a of the light guide plate 512.
A portion of the FPC 507, which is disposed on the back surface side of the frame 515, is bonded to the frame 515 by use of an adhesive member 517 made of a double-sided adhesive tape or the like.
The light guide plate 512 is formed so that the light incidence surface 512a and the light output surface 512b are substantially orthogonal to each other. Furthermore, an inclined surface 512c inclined at an angle θ 101 (e.g. 6°) with respect to the light output surface 512b is formed between the light incidence surface 512a and the light output surface 512b of the light guide plate 512. That is, the light guide plate 512 is formed so as to have a thickness larger at a portion on the side of the light incidence surface 512a (side of the LEDs 511) than at a portion on the side of the light output surface 512b. 
Furthermore, the light guide plate 512 is disposed at a predetermined distance from an after-mentioned adhesive member 518.
The plurality of optical sheets 513 are composed of a diffusion sheet 513a, a lower lens sheet 513b, and an upper lens sheet 513c. 
The frame 515 is bonded to the display panel 502 by use of the adhesive member 518 made of a double-sided adhesive tape or the like.
In the backlight device 510, as described above, the light guide plate 512 is formed so as to have a thickness larger at the portion on the side of the light incidence surface 512a (side of the LEDs 511) than at the portion on the side of the light output surface 512b, and thus an increased proportion of light outputted from the LEDs 511 can be made incident on the light guide plate 512.
Furthermore, the light guide plate 512 can be made thinner at the portion thereof on the side of the light output surface 512b, and thus the backlight device 510 (display device 501) as a whole can be reduced in thickness.
Furthermore, in the backlight device 510, since the light guide plate 512 is disposed at a predetermined distance from the adhesive member 518, it is possible to prevent light incident on the light guide plate 512 from being outputted to the side of the adhesive member 518 to be absorbed by the adhesive member 518. If the light guide plate 512 is bonded to the adhesive member 518, light incident on the light guide plate 512 is likely to be outputted from a portion of the light guide plate 512 where the light guide plate 512 is bonded to the adhesive member 518. Light outputted from the light guide plate 512 then is absorbed by the adhesive member 518. Because of this, if the light guide plate 512 is bonded to the adhesive member 518, light use efficiency is deteriorated, which leads to a decrease in the luminance of the display panel 502.